Encoder Principles: How is Position Established?

http://zone.ni.com/devzone/cda/ph/p/id/132

Dear Vulcan,

Don't get angry.We are having both Absolute and incremental encoders in our CNC machines.We are facing more problem in incremental type encoder system,problems like home position limit switch failure.This consume more time for restore the machine.But in the absolute ,It not create such type of problems.So that only I had posted my doubt and I like to clarify myself.

Thank you,

cskt.

I didn't see anything to be angry about. You asked how they worked so I gave you a link (making my own explanation will take too long).

I've used both absolute and incremental encoders and I don't get what you mean by home-position-limit-switch. The encoders I used do not have internal limit switches. You must be talking about an external limit switch.

In my case, I used absolute encoders for angular positioning and incremental for linear timing. Homing for each type is different.

In an absolute encoder, our system memorizes the position that it considers as "home". When returning to home position, the servo turns towards it, and at a certain point before it arrives, it decelerates until it reaches the home position at which point, it brakes. We have a home limit switch but it's located a small distance from home. If the servo misses home and goes past it, hitting the switch will switch off the system and generate an alarm. The limit switch is designed so that it won't be damaged if the thing hits it.

In an incremental encoder application, we use it for timing when events start or stop. We have a conveyor system where we put objects that need to be precisely positioned. As the conveyor runs, things are done to those objects as they pass the different stations. Think of it this way:

You have a sandwich making process. At the first station, the first slice of the sandwich is placed. At the second station, the spiced ham slice is placed. At the third station, we apply mayonaisse. Finally at the fourth station, the last slice of bread is placed.

All those things need to happen at exactly the right time and place, otherwise you have spiced ham and mayonaisse on the conveyor instead of the bread.

Note that I didn't mention any home position. That's because there isn't any. What I would consider as home is the placement of the initial slice of bread at the first station. All things happening later is timed from that event.

Of course, your application is probably different from ours (we don't have a sandwich-making machine by the way. That was just an example). Without knowing exactly what your situation and problem is, I cannot offer exact solutions.

Hi Vulcan, I like your posts, they demonstrate that you are a practical person as well as a good theory person, both are needed in this world!

You and I are hearing from several other sources about limit switches, also problems with them as they are really unreliable, which is why I mentioned in a post of mine, that well designed and programmed electronics of this type does/should not use them anymore.

Vulcan was also put slightly on the wrong foot in this area (as I would have been too in his position), it is sad that so many design engineers are still designing limit switches into hardware completely unnecessarily.....

Now do the rest of you (not Vulcan, he knows why!) understand why?

The encoder explanation give by a previous poster is excellent and gives good infos. If you want to use this, then read on here for further movement infos using encode output!

The way to use such encoder outputs in software is to reset everything at power on, then tell the motor to move slowly in one direction until a so called "crash stop" is reached.(set by yourself at points beyond the maximum end travel that you wish to achieve. Both ends of course is best).

You have a counter running from the encoder as a sort of over or past safety limit, that is a count that once exceeded shows that something is totally wrong, you will never get to the stop as you are already say 1000 counts beyond what ever the max might be. If this count is reached, deactivate and post error code.

You detect the crash stop today by monitoring motor current (it used to be end or limit switches, not a good idea!), when the motor current goes above a certain value (stalled motor) you assume one end stop position.

Then you start counting from the encoder outputs as you travel in the opposite direction till the other end stop is found. Now you have the relative positions of each end stop....you know exactly where you are!!

Now you can use a higher speed to move around....but between each end stop, without ever reaching them again.....

Greetings,

You asked:

what is the working principal of encoder?how it read the position,how it keeps the home position?

I have worked on milling machines which used incremental rotary encoders. The encoders were rotary and produced two circular quadrature pulse strings. Since the pulses overlapped by 90 degrees the servo could easily tell direction by which pulse occurred first. In addition there was a single reference pulse produced each rev. When the servo was enabled the control sent it home which required that a limit switch make and the reference pulse be detected. When this occurred the control stopped the servo and declared "home" reached. The control had already been programmed to know how many pulses where needed to develop full travel so from then on all that was required was counting pulses. A neat thing also, backlash was programmed out by measuring it and telling the machine how many counts to correct by when it reversed direction.

I have very little experience with absoulute encoders.

Hope this helps. Best wishes!

Greetings!

what is the working principal of encoder?how it read the position,how it keeps the home position?

Additional thought overnight: The "homing" process on the machines I knew went like this: When the operator asked the machine to "Calibrate" which I refer to as "home" the servo was automatically driven in a fixed direction. When a limit switch or "end-of-travel" sensor was activated, the servo automatically slowed to a creep while hunting for the "marker" or "reference" pulse from the encoder on the motor shaft. When that was detected the control stopped, cleared the position counter and declared itself "home". We never wanted to make a "hard stop", deliberately anyway, but the home position was very near hard stop to preserve max travel. Now what is important is that the marker pulse from the encoder not be too close to the detection of the limit switch. Some limit switches were really magnetic proximity detectors and would pick up chips over time so any foreign material that would change the relative position of the limit switch is bad news. Don't try and cause the marker pulse to be too close to the actual initial detection of the limit switch because such prox dectectors are dirty and change a little over time. Better to give the control some room between switch detection and marker detection.

Trying to help and not confuse, best wishes.

You have very clearly stated the case for NOT using limit switches, I thank you most kindly......

They are never needed in a correctly designed and programmed unit. I used to work in Robotics and the company I worked for gave up using limit switches in the early 80's (as many others did, they were not alone!!)

If you need more info (my other Posts have it already), just ask.

Thank you kindly for your comments,

You have very clearly stated the case for NOT using limit switches, I thank you most kindly......

They are never needed in a correctly designed and programmed unit. I used to work in Robotics and the company I worked for gave up using limit switches in the early 80's (as many others did, they were not alone!!)

If you need more info (my other Posts have it already), just ask.

I can only comment about machines with which I have worked. Milling machines cannot home a heavy head or table weighing perhaps several hundred pounds at max servo speed against a hard stop without doing damage or upsetting the machine mechanically. There are thousands of milling machines operating using the procedure I described for calibrating (homing) all three axes. I cannot comment about your field of expertise. I do recall that other than component failures we did see several machine problems caused by customer maintenance people who did not understand the process. They would see the axis creeping after the limit switch was detected and assume a problem. Consequently they either unbalanced the servo or tried to move the limit switch to sync with the ref pulse on the encoder thereby causing the machine to home erratically afterwards. Ideally the ref pulse must occur some time after the limit or prox is set to account for the sloppiness of such devices.

Thank you again for replying. Once again, let me say that I am not trying to disagree with your comments regarding robotics, I can only offer what I am familiar with.

Very fair, no problems with that.

We were swinging robotics accelerating and braking them at up to 30 feet a second, of parts that weighed over 500 Lbs........there are not that many companies that can do that I suppose, though I never thought much of it as I used to work on MK44 4.5 inch Gun turrets and we swung those around at high speed and a complete turret weighed in at about 44 tons if I remember correctly, but it was hydraulic, not electric directly....designed in the early 50's or late 40's I believe.

But if you re-read my original post about the non usage of limit switches, I did mention that when a machine is switched on, it would need to discover the "crash stops" at a very low speed first. Also, a "Crash stop" does not need to be completely solid, it can be loaded with a very heavy spring, so that the motor current will rise and be detected, without causing any damage to anything...

The accuracy of detecting the motor current is important and not even difficult as when running slowly, motor current is relatively small and the crash stop resistance immediately causes the current to be easily 4 x that low current, eg. very detectable with relatively simple and common methods....

These current detectors are still usable (as they are not only working at a fixed value!) when the machine is running, even though much higher bursts of current are used, the firmware will know if and when even then that something is wrong (provided it is correctly set up of course to do that).

So you can use it to help protect heavy and expensive motors/equipment in the event of a catastrophic failure....

Limit switches are for me a further problem when working in an unfriendly environment with explosive gases or fluids are everywhere....so not having them at all can be a big plus.

I wish you a great day and thanks for the friendly discussion.

We use encoders for various applications here. (compression molding small plastic parts) once the home position is establishe on one of our molders (3 encoder system;1- servo, 1- rotating carousel, and 1 reference for timing the servo in relationship to the speed) the home position is then stored in the servo controller and on an EEPROM (for future reloading) Once the servo (encoder is integral to the servo motor) reaches it's home position then the controller sends a signal to the VFD controlling the carousel, to rotate the carousel to the "home" position. Both of these "home" positions must be reached or the equipment will not and ask you to zero axis. The program in the VFD has been copied to several HMI's in order to facilitate reloading if necessary. There are no overrun L/sw's as it is a rotating carousel.From the home(zero axis) position the encoders act as pulse generators in this application.